To start with, one has to know the MAJOR limitations of talc-based filler products: for one, filler should never be used to "fill" anything - only to smooth the final imperfections after metalwork has been properly performed. Second, talc-based fillers are NOT WATERPROOF - and as such, should never be W usedin an area or space that may get wet, or could hold moisture - and thirdly, it is not structural! It won't "hold" anything together, won't re-enforce anything and will definitely not flex.
For those last 2 items - waterproof and flex - the correct product to use is a fiberglass reinforced product. It comes in both short and long strand mixes and uses plastic resins to bind instead of talc. This product should be used over any welds, to "fill" anything deeper than 1/8 inch or if some partial re-enforcement is required. Again, it's not to be used in place of proper metalwork, but is a key product in bringing that fine metal work to its finished stages.
The other key consideration to filler use is proper panel prep. Many guys will just run some coarse paper over the area (or grinder) and slap on the filler when these products are meant only to go on to properly prepared metal.
First step in preparing metal for filler is to treat any rust with a phosphate rust-converting product. My favourite has long been from POR15, which is a non-toxic phosphate wash. This product converts Iron-Oxide (Rust) to Iron-Oxite - a black hard substance that doesn't spread, which means no bubbles will be popping through your expensive paint job years down the road!
The wash chemically treats both the rust and all the metal for maximum
corrosion protection and adhesion:
Once the wash has sat overnight, it is cleaned off with a solvent wipe, which also degreases,
removes wax and dirt residues and ensures the perfect substrate for any filler work.
Time for many hours of block sanding !